happyyoudaoicibaDictYouDict[happy 词源字典]
happy: [14] The Old and Middle English word for ‘happy’ was what in modern English has become silly. This began to change its meaning around the 15th century, and obviously an opportunity began to open up for an adjective expressing ‘contentment’ (as opposed to positive ‘joy’, denoted then by glad, fain, and joyful). The gap was partly filled by a weakening in the meaning of glad, but waiting in the wings was happy, a derivative of the noun hap ‘chance, luck’ (source of happen), which when it was coined in the 14th century meant ‘lucky, fortunate, prosperous’.

The main modern sense ‘highly pleased or contented’ developed in the early 16th century.
=> happen[happy etymology, happy origin, 英语词源]
happy (adj.)youdaoicibaDictYouDict
late 14c., "lucky, favored by fortune, being in advantageous circumstances, prosperous;" of events, "turning out well," from hap (n.) "chance, fortune" + -y (2). Sense of "very glad" first recorded late 14c. Meaning "greatly pleased and content" is from 1520s. Old English had eadig (from ead "wealth, riches") and gesælig, which has become silly. Old English bliðe "happy" survives as blithe. From Greek to Irish, a great majority of the European words for "happy" at first meant "lucky." An exception is Welsh, where the word used first meant "wise."

Happy medium "the golden mean" is from 1702. Happy ending in the literary sense recorded from 1756. Happy as a clam (1630s) was originally happy as a clam in the mud at high tide, when it can't be dug up and eaten. Happy hunting ground, the reputed Indian paradise, is attested from 1840, American English. Happy day for "wedding day" is by 1739; happy hour for "early evening period of discount drinks and free hors-d'oeuvres at a bar" is by 1961, said to be 1950s. Related: Happier; happiest.
happy (adv.)youdaoicibaDictYouDict
late 14c., from happy (adj.).